1. Field of the Invention
The present invention generally concerns electronic voltage regulators, such as those found in motor vehicles, that regulate the current in the field winding of an alternator in accordance with the level of a direct current, battery, voltage in accordance with a rectified voltage that is maintained by the alternator.
The present invention particularly concerns improvements to the electrical circuits of electronic voltage regulators which improvements are directed to protecting the regulators against (i) induced failure due to overload, and also, separately and additionally, (ii) induced failure due to loss of a reference voltage.
2. Background Art
Electronic voltage regulators are used with alternators in vehicular and other applications. A regulator applies a pulse-width-modulated voltage signal to the field winding of the alternator. The alternating current voltages correspondingly induced in the remaining windings of the alternator, normally three in number, are rectified, normally in a full bridge rectifier circuit, to produce a direct current. This direct current is applied, in parallel with the vehicle's battery, to power the electrical devices of the vehicle, and to maintain the charge of the battery.
The voltage regulator senses the battery voltage, or "B+", of the vehicle. It attempts, by variation in the duty cycle of the pulse-width-modulated output signal, to maintain this battery voltage constant. It so functions independently, within certain limits, of the electrical load on the battery voltage and/or the rotational speed of the vehicle's alternator.
Modern electronic voltage regulators, like most solid sate electronic devices, are intrinsically reliable; exhibiting a low failure rate when electrically connected, and used, as intended. However, electrically mis-connected or incompletely-connected voltage regulators can be subject to induced failures. Circuits are known in the electronic arts for the protection of medium- and high-power electronic devices against induced failure(s) due to over-voltage or over-current. However, most of these circuits are not readily adaptable to electronic voltage regulators. Moreover, because economy is desired in the construction of voltage regulators (which use very few components, and commonly as few as two transistors), these existing circuits generally seem too cumbersome and expensive to adapt for the electrical protection of electronic voltage regulators.
Accordingly, electronic voltage regulators, and especially those mass-produced for vehicular applications, are generally substantially internally unprotected against induced electrical failure. This is true even though the electrical environment in which such voltage regulators are used, and in which replacement voltage regulators are installed, is often hostile to the electrical integrity of the voltage regulators. The environment is so hostile as regards electrical phenomena (i) that are induced, such as lightning, (ii) that occur, over time, due to wear and deterioration of a vehicle's wiring and connectors, such as shorts and opens, and (iii) that result from electrical misconnection or cross-connection or selective non-connection due to human error.
The (i) climatic electrical adversities faced by a vehicular voltage regulator are wide-ranging and often severe, but thankfully rare. A vehicle within which a voltage regulator resides may be subject to electrostatic discharge (ESD) from lightning strikes or from contact with power lines. The immunity or tolerance of an electronic voltage regulator to (i)climatic electrical adversities is generally not the subject of the present invention. However, the circuits and circuit improvements of the present inventions will be seen to be both (i) totally compatible with proper design considerations regarding the climatic environment of use, and (ii) beneficial in avoiding induced failures resulting from such electrical rigors as naturally occur.
The major (ii) induced adverse electrical phenomena commonly encountered by a voltage regulator result from a short circuit, or partial short circuit, in the field winding of the alternator to which the voltage regulator is connected, or in the electrical connections to this field winding. Neither of these shorts are rare; both are regularly encountered during the decades-long life of an automotive vehicle. The output power transistor of the voltage regulator is normally unprotected against catastrophic overcurrent, and failure, from driving into a shorted load. A short in the field winding of an alternator generally results from a build-up of carbon dust, which build-up is especially likely to be troublesome when the groove between the slip rings of the alternator's shaft has worn to a shallow depth.
Because modern alternators of high quality are long wearing, in many vehicular applications it is probably equally or more likely that the electrical connection from a voltage regulator to the alternator will become shorted -- inducing a catastrophic failure of the output transistor within the voltage regulator -- as it is that the alternator will short internally. Avoidance of this failure mode is one reason that modern voltage regulators are often packaged integrally with the alternators that they regulate.
However, whether a voltage regulator is packaged separately from or integrally with an alternator, it is still subject to (iii) manually-induced electrical stresses. In fact, the greatest single environmental nemesis of a voltage regulator may be a human.
It is generally possible, and even desirable, for a human to obtain access to the output terminal of the voltage regulator -- which terminal is connected to the field winding of the alternator -- for the purposes of diagnosis, and modular repair, of failures occurring within the charging system of the vehicle. Historically, many mechanics of lessor skills or training have learned a "quick and dirty" way of confirming the presence of a voltage output from the voltage regulator, and within the field coil of the alternator, for the diagnostic purpose of isolating a charging system failure to the voltage regulator or to the alternator. This "quick and dirty" diagnostic method is to momentarily short the output terminal of the voltage regulator with a grounded screwdriver or the like, drawing an arc or spark. Some older mechanics even learned this technique as being sound practice in the bygone days of mechanical voltage regulators and generators. Alas, the very arc that indicates that an electronic voltage regulator is correctly functioning (at least to the extent of producing a voltage in the alternator's field coil) may induce its catastrophic failure.
Still another major failure mode of an electronic voltage regulator arises from human error. This failure mode of a voltage regulator occurs when the voltage reference to the voltage regulator is lost while the remaining voltage connection to the voltage regulator, and the voltage regulator's connection to the field coil of the alternator, are maintained. In an embodiment of an electronic voltage regulator where its bipolar PNP output transistor is connected between a positive, B+, battery voltage and the alternator's field coil, the loss of the voltage reference occurs when the remaining, ground, connection to the voltage regulator is lost. In an alternative embodiment of the electronic voltage regulator where its bipolar NPN output transistor is likewise connected (in the opposite sense) between ground and the alternator's field coil, the loss of the voltage reference occurs when the remaining, B+positive battery voltage, connection to the voltage regulator is lost.
When its voltage reference is lost the voltage regulator has no differential voltage upon which to base its output. It proceeds to "chase a ghost" as its rising voltage output produces no observable change in the battery voltage sensed. The voltage regulator "runs away" into a constant "on" condition of its output transistor, which subsequently fails from overcurrent.
This additional failure mode can arise from manual manipulation of the electrical connections to the voltage regulator, or from corrosion on the reference voltage terminal of the voltage regulator, or from any other cause of a loss of the integrity of the reference voltage connection. Most commonly, however, this loss of electrical integrity results when a careless, inattentive, or ignorant mechanic fails to tightly secure the case of the voltage regulator to the chassis ground of the vehicle.
According to this range of human maintainer-induced problems, a voltage regulator that is sold in the automotive aftermarket for installation by amateurs leads a hazardous, and failure-prone, existence. This fact is reflected in returns from the end item consumer of the voltage regulators that are alleged to have been defective on delivery, or to have failed in their infancy. Although the manufacturer of the voltage regulator often knows from the installation of the same voltage regulator in new vehicles, and/or from his quality and failure analysis programs, that the premature failure of the voltage regulator was very likely induced by its installer-purchaser, it is necessary to maintain a liberal return policy, accepting back for full credit all returned units, in order to secure consumer and retailer goodwill.
Accordingly, it would be very useful to a purchaser-installer, and a user (whether performing the installation or not), of a voltage regulator if some of the failure modes of the voltage regulator could be abated, and if the reliability of the voltage regulator increased. Although the purchaser-installer or user might neither recognize nor appreciate such increased reliability, it would inure to their benefit. Additionally, it would also be of immediate interest and advantage to a manufacturer of voltage regulators, especially of such voltage regulators as are sold as repair parts in the automotive after-market, if the rate of infancy failures of the units at the hands of the purchaser-installers could also be improved.
In still another area of the present technology of electronic voltage regulators, it is known that the junction size, current capacity, and rating of the output transistor -- a major, typically 40%, cost driver in the entire electrical circuit of the regulator -- can be minimized for a particular application rating of the voltage regulator if its bipolar output transistor is overdriven at its base junction, thereby permitting the transistor to operate more efficiently.
It may be correctly surmised that the circuits of the present invention -- involved as they are in protecting a voltage regulator and its output transistor from catastrophic failure due to (i) a short circuit load and/or (ii) loss of a reference voltage -- will somehow involve turning the output transistor "off" before it is destroyed. It would be very useful if this improvement in reliability could be accomplished without appreciably adversely affecting any other criteria of the voltage regulator's performance, particularly including its life cycle cost of ownership and operation. It would be particularly useful if any enhancements to the circuit of the voltage regulator did not degrade any normal overdrive of its output bipolar transistor, and did not adversely affect the cost or operational efficiency of this or of any other component within the voltage regulator.